def Isprime(a):
    if a == 1:return False
    test = True
    k = a-1
    while k>1:
        if not a%k:
            test = False
            break
        k-=1
    return test

n = input()
r=[]
for i in range(len(n)):
    for k in range(i+1,len(n)+1):
        a = int(n[i:k])
        if Isprime(a):
            r+=[a]
print(max(r) if r else 0)